Outer Limits 4x4 Board

hi guys ive got an 80 series running on lpg runs perfect on petrol but when its switched to gas it revs ok and runs but when ive been driving at a cruising speed and then back off the accelerator it wants to stall and lerches back and forth thus chewing out my cv"s ive had it back to the mechanic on 4 occasions hes had it on the emission tester and made some adjustments but its still doing it. anyone got any ideas of what it could be hes already changed some part that adjusts the flow to the air intake still no fix. he also tryed taking out the blowback valve as he thought it might have been too small and wasnt letting enough gas through properly but still no fix either. im thinking maybe the wiring box might be stuffed that controls it im getting a bit desperate now thanks muds